Injection of additives such as microbubbles or high molecular weight materials such as polymers into the boundary layer of a fluid flow has been shown to reduce skin friction drag significantly for both vessels moving relative to water and for pipeline applications. The microbubbles or large polymer molecules interact with the turbulent activity in the near-wall region, absorbing energy and reducing the frequency of burst (high energy fluid moving away from the wall) and sweep (low energy fluid replacing the high energy fluid in the near-wall region) cycles. The reduced burst frequency results in less energy dissipation from the wall and can result in skin friction drag reductions up to 80%. Experiments have shown that the efficacy of polymer to molecules for drag reduction is closely related to their molecular weight, their location in the boundary layer, and the degree to which they have been unwound, aligned, and stretched (i.e., xe2x80x9cconditionedxe2x80x9d).
In the past, polymer mixture ejectors have been simple slots that ejected a mixture/solution of polymer and a fluid at an angle to the wall. To attain high drag reduction for a reasonable distance downstream with this ejection approach, large quantities and high concentrations of polymers must be ejected in order to flood the entire boundary area, creating a xe2x80x9cpolymer oceanxe2x80x9d effect in the boundary layer. The high polymer consumption rates of these systems have made them impractical for many drag reduction applications.
Fluids containing other substances than high molecular weight materials, e.g. microbubbles, surfactant, etc., as well as fluids with or without additive which are heated so as to achieve a lower viscosity in the viscous sublayer, have been used in prior art attempts to reduce surface friction drag. These, however, each require very large amounts of additive or quantities of heated fluid. To be useful for practical applications, a more efficient method for ejecting additive(s) or fluids, such as low viscosity or heated fluids, for drag reduction needs to be devised.
Another method to reduce skin friction is to impose on the surface narrow microgrooves, commonly referred to as riblets. The riblets also affect the development of turbulent structures in the near-wall region and thereby reduce the burst frequency. While the technique does not require the expenditure of consumables such as polymers, the level of drag reduction is more modest than with additives, usually about 6 to 12 percent. Further, such systems are not effective for marine surfaces that are exposed to microfouling from biological slimes. Since slimes begin to form in a matter of hours, riblets are only practical for marine surfaces that can be regularly cleaned. Thus, a technique to affect the development of turbulence which does not require the expenditure of additives but which is tolerant of the marine environment also will be useful even if the effect is modest relative to polymer drag reduction.
The present invention enables the efficient ejection of fluid mixtures/solutions containing a drag-reducing additive or additives into the near-wall region of a boundary layer of a fluid flow in order to reduce drag. A first object of the invention is to enable drag to be reduced for a first fluid moving relative to a surface by ejecting a second fluid into the near-wall region of the boundary layer of the surface such that the second fluid retards or inhibits the xe2x80x9cburstxe2x80x9d and xe2x80x9csweepxe2x80x9d cycles, as discussed above. The second fluid may or may not: (1) be the same fluid as the first fluid, (2) be of lower viscosity (e.g., elevated in temperature) relative to the first fluid, and/or (3) contain one or more drag-reducing substances as an additive in mixture or solution. A second object of the invention is to release a drag-reducing substance only into the near-wall region of the boundary layer, by ejecting it substantially parallel with the streamlines of the boundary layer. A third object of the invention is to extend the time the drag-reducing substance is operative in the near-wall region of the boundary layer by creating low pressure regions immediately adjacent the wall. When a drag-reducing substance is added to the second fluid, the drag-reducing substance may comprise a mixture/aqueous solution of high molecular weight polymer, surfactant, gas micro-bubbles, or any combination thereof. When the additive includes polymer, a fourth object of the invention is to condition the polymer molecules prior to ejection so that drag reduction occurs almost immediately upon ejection into the first fluid.
When a drag-reducing additive that includes polymer is used, the present invention preconditions the drag reducing mixture/solution for improved drag reduction performance using a unique arrangement of flow area restrictions, as well as by employing dimples, grooves and elastomeric materials. The dimples, grooves and flow area restrictions are sized relative to one another and to the Reynolds number of the flow for optimal polymer molecule conditioning (unwinding, aligning with the flow and stretching) so as to provide optimal drag reduction after ejection into the fluid flow.
The present invention uses a new approach to ejecting the second fluid so that it minimizes disruption of the boundary layer of the first fluid. This aspect of the invention employs a convex Coanda surface on the downstream side of a slot ejector and controls the ejected velocity so as to be a small fraction of the flow velocity of the first fluid past the surface, thereby enabling the stream lines of the second fluid to be nearly parallel with the streamlines of the first fluid.
The present invention also uses a new approach to structuring the flow in order to reduce migration/dissipation of the second fluid away from the near-wall region of the downstream wall. This is achieved by one or more ejectors, each having a carefully designed chamber under the slot. The upstream wall of the chamber has the surface curvature and features that establish a duct-like system of longitudinal (i.e., in the direction of the flow) Gxc3x6rtler vortices. Gxc3x6rtler vortices are formed by the centrifugal effect of a fluid flow that is given angular velocity by a concave surface. The duct-like system of Gxc3x6rtler vortices formed by the present invention mimic the spacing, but have the opposite rotation, of the naturally occurring quasi-longitudinal vortex pairs in the boundary layer. The pairing of naturally occurring quasi-longitudinal vortex pairs is such that they migrate from the wall and are believed to contribute to the development of bursts and sweeps that account for a large portion of hydrodynamic drag. The vortex pairs, created by the upstream wall of the chamber, have an inverted orientation relative to the downstream wall and thus opposite signs of rotation that cause the vortex pairs to hug the downstream wall as they pass along it after being shed from the chamber. This advantageously causes the second fluid or drag-reducing substance that has been ejected into the near-wall region of the boundary layer of a first fluid flowing by the wall to remain in the near-wall region. It also enables drag reduction to be achieved (by reducing the frequency of burst and sweep cycles) when the second fluid contains no additives, is identical in temperature, and is the same type fluid as the first fluid.